Emerald Pool is the name of several locations.
Emerald Pool is a small, shallow lake located immediately above Vernal Fall in Yosemite National Park. It is named for its deep green color, which is caused by algae living on the rocks at the bottom of the pool. In the summertime during diminished water flow, the Silver Apron (a smooth granite slope over which the Merced River flows into the Emerald Pool) is frequently (albeit illegally) used by hikers as a water slide. Swimming or wading in the Emerald Pool or entering the Silver Apron is prohibited by the National Park Service as waders or swimmers have been swept over Vernal Fall. This prohibition is clearly marked with signs.
There also is an Emerald Pool in Northern California along the Yuba River. The Yuba River flows down a canyon then falls down a water fall into a shear rock canyon around 60 feet. The walls around the first pool measure from around 30 feet to 80 feet in height with a length of around 50 meters and a width of around 30 meters, the depth is around 7–10 meters, the water is crystal clear and many people jump off the cliffs for enjoyment. There is a second pool nearby, not as big, but close. It is popular with young adults from Reno and Grass Valley areas. It is a popular swimming spot although the water is cool.
Other locations named Emerald Pool are in Morne Trois Pitons National Park, in Dominica; in Yellowstone National Park; and in Zion National Park in Utah.